1. Field of the Invention
The present invention relates to analog communication systems. In particular, the present invention relates to analog communication systems, such as telephony systems, wherein analog control signals are sent in voice band.
2. Description of the Related Art
An analog communication system is a communication system wherein information is transmitted as a continuous-time signal. One such analog communication system is an analog telephony system.
Within analog telephony systems, it is necessary for communication system control information to be transmitted between communications devices. Such information can be used to control switching, to alert a telephone user that he or she has a call waiting, or to display caller identification information. One way in which such control information is transmitted within telephony systems is referred to as xe2x80x9cin-band signaling.xe2x80x9d
In an analog telephony system, analog electrical signals utilized within the telephony system are directly translated to mechanical signals of identical frequencies by use of a transducer such as a telephone speaker. The mechanical signals from the speaker are perceived as sound xe2x80x9ctonesxe2x80x9d by a human telephone user. The electrical signal frequencies which directly translate to mechanical signals (tones) which humans can hear are referred to as xe2x80x9cvoice bandxe2x80x9d frequencies, since what a human usually hears over a telephone is the human voice.
When control information signals are sent in the range of electrical frequencies which will translate to tones humans can hear, such signaling is referred to as xe2x80x9cvoice-bandxe2x80x9d or xe2x80x9cin-band signaling.xe2x80x9d One specific scheme in which this is currently done is known as Dual Tone Multi-Frequency (DTMF) signaling. xe2x80x9cDual Tonexe2x80x9d implies that any two frequencies used will be in-band, Multi-Frequency implies that more than one frequency per tone is possible.
A problem which commonly occurs when in-band signaling tones, single or multiple,are used to transmit information, is the accurate estimation of the time duration for which the signal is present. For example, in DTMF tones used in standard telephony applications for push-button signal reception, signal durations in the range 20-40 ms are specified as the minimum-to-maximum requirement in various National and International Standards. Outside this range, the signal can be accepted or rejected as a true signaling tone, depending upon the accuracy of measurement specified by a particular Communications Administration.
The current trend is for more and more information to be conveyed via DTMF signals. A relatively recent example of such increase in information to be conveyed via tone detection is the group of services known as Caller Identity Deliver on Call Waiting (CIDCW) which provides caller identity information to the subscriber for calls that are call waited.
As more and more information is conveyed via DTMF signals, the allowable signal tolerances are dropping. That is, system performance requirements are rising. One example of such rising performance requirements is set forth in Bellcore Special Report SR-TSV-002476, Issue 1, December 1992 which sets forth a series of six very stringent performance criteria, all of which must be passed for DTMF systems to be deemed xe2x80x9cBellcore Standard Compliant.xe2x80x9d
One set of xe2x80x9cBellcore Standard Compliantxe2x80x9d performance criteria relates to tone detection. A Bellcore Compliant system must be able to detect tones within a very narrow frequency tolerance, and a tightly controlled time window. For example, for the two DTMF tones 2130 Hz and 2750 Hz, the Bellcore performance criteria are as follows: Lower Tone: 2130 Hz+/xe2x88x920.5%; Upper Tone: 2750 Hz+/xe2x88x920.5%; Dynamic Range: xe2x88x9214 to xe2x88x9232 dBm per tone; Power Differential within Dynamic range: 0 to 6 dB between tones. The specified duration for the DTMF signal to be accepted as a control signal by Customer Premises Equipment must fall within a time window 75 to 85 ms duration.
Detection of such signals in environments where noise or speech is present is generally recognized to be a complex issue. One such complexity involves the issues of talk-off and talk-down.
Talk-off occurs whenever a signal tone detector erroneously accepts signal imitations, such as those produced by speech or music, as valid DTMF signals. These signals can imitate some of the temporal and spectral characteristics of DTMF signaling tones. These imitations are likely to trigger, or xe2x80x9ctalk-offxe2x80x9dxe2x80x94the xe2x80x9ctalkxe2x80x9d setting xe2x80x9coffxe2x80x9d, signal detectors. An important goal in designing such detectors is making them immune to these signal imitations.
Talk-down occurs whenever a signal detector erroneously rejects valid DTMF signaling signals due to the presence of competing speech, music or other extraneous background noise. The existence of these complex signals introduces spectral components into the signal that distort and ultimately impair the detection of valid DTMF signaling tones. This is analogous to a situation in which a person is xe2x80x9ctalked downxe2x80x9d by another person shouting at him or her, thus this situation is referred to in the art as xe2x80x9ctalk-downxe2x80x9d. A signal detector is said to have been xe2x80x9ctalked downxe2x80x9d whenever it fails to recognize valid signaling tones that were masked by speech, music, or noise.
One group of services known as Caller Identity Deliver on Call Waiting (CIDCW) requires reliable signaling in an adverse signaling environment. The essence of this service is to provide caller identification information to the subscriber for calls that are call waited.
Imagine that two parties, a near-end and a far-end party, have a connection established between them. Suppose that the near-end party subscribes to CIDCW. Further suppose that a third party attempts to call the near-end party. Upon completion of dialing the near-end party""s number, the third party receives audible ringing. The Central Office (CO) switch recognizes that a call is destined for the near-end party and starts to execute the CIDCW service routine. The switch splits the connection and, in consequence, mutes the far-end party. It sends the regular call waiting signal, a burst of about 300 msec of a 440 Hz tone, to the near-end party and it appends to this call waiting signal a short burst of a special alerting signal, a Customer Premises Equipment Alerting Signal (CAS) which is intended to prompt the near-end party""s equipment. This equipment must reliably detect this alerting signal. Upon reception, the subscriber""s handset and any other parallel extension handset is muted, an acknowledgment signal is sent back to the CO and the subscriber""s equipment places a Frequency Shift Keying (FSK) data receiver on the line awaiting the caller identification information. The near-end party""s equipment then receives the data, decodes the information and displays it for the subscriber to view. The connection between the near- and far-end party is then re-established once data transmission is complete.
It can be seen that the degree to which this service works depends on the accuracy with which the alerting signal is detected by the subscriber""s equipment. Since the signaling scheme chosen for this service was in band dual tone signaling, the problems of talk-off and talk-down are now relevant where the consequences of failure can be as follows.
A first consequence relating to talk-off is that if the subscriber""s detector is talked off and incorrectly accepts a signal imitation produced by speech, the Customer Premises Equipment (CPE) will interrupt the connection by muting the handset, and any extension handsets, and will send back an acknowledgment signal at a relatively high amplitude in comparison to what the subscriber would normally hear on the line. The connection between the near-end and the far-end will remain interrupted until the CPE times out waiting for data. Since the CO did not originate the alerting signal, the far-end party will unintentionally receive the acknowledgment signal at an undesirable listening level.
Alternatively, a second consequence relating to talk-down is that if the CPE is talked down and fails to recognize an alerting signal sent by the CO, no caller identification information will be delivered and the service paid for by the subscriber will not be rendered.
In both cases, detector failure degrades the quality of the service. Thus, since both talk-off and talk-down have negative consequences, the presence of such occurrences must be minimized. Also, because the alerting signal can be sent at any time while the near-end and far-end parties are connected, the CAS detector must remain on the line for the entire duration of the call. Therefore, during this time, the detector is constantly exposed to speech with the consequent possibility of talk-off. Since talk-off degrades the voice path, repeated talk-offs in relatively short time periods should be avoided.
As has been discussed, the phenomena of talk-off and talk-down have deleterious effects upon communication efficiency. As has also been discussed, the cause of the talk-off and talk-down phenomena is the inability to accurately the detect the presence of DTMF signals in a noisy environment. It is therefore apparent that a need exists for a method and system which will substantially maximize the detection of dual tone multi-frequency signals, particularly in an environment where other signals, such as music, speech, data, and noise can be simultaneously present with the dual tone multi-frequency signals.
It has been discovered that a method and system can be produced which will substantially maximize the detection of dual tone multi-frequency signals, particularly in an environment where other signals, such as music, speech, data, and noise can be simultaneously present with the dual tone multi-frequency signals. In the method and system, an analog signal is received. A stream of data samples is created from the analog signal. Based on the stream of data samples, a confidence factor regarding the presence of the one or more analog signals is created. The confidence factor is created based upon a confidence factor calculated for each of the one or more analog frequency signals. The confidence factor calculated for each of the one or more analog frequency signals is calculated utilizing incremental energies associated with each of the one or more analog frequency signals.
The foregoing summary is illustrative and is intended to be in no way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.